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March 12, 2010

Einstein’s Theory Applies Beyond The Solar System

by Sam Savage

A team led by Princeton University scientists has tested Albert Einstein's theory of general relativity to see if it holds true at cosmic scales. And, after two years of analyzing astronomical data, the scientists have concluded that Einstein's theory, which describes the interplay between gravity, space and time, works as well in vast distances as in more local regions of space.

The scientists' analysis of more than 70,000 galaxies demonstrates that the universe -- at least up to a distance of 3.5 billion light years from Earth -- plays by the rules set out by Einstein in his famous theory.

Ever since the physicist Arthur Eddington measured starlight bending around the sun during a 1919 eclipse and proved Einstein's theory of general relativity, the scientific world has accepted its tenets. But until now, according to the team, no one had tested the theory so thoroughly and robustly at distances and scales that go beyond the solar system.

Reinabelle Reyes, a Princeton graduate student in the Department of Astrophysical Sciences, along with co-authors Rachel Mandelbaum, an associate research scholar, and James Gunn, the Eugene Higgins Professor of Astronomy, outlined their assessment in the March 11 edition of Nature.

Other scientists collaborating on the paper include Tobias Baldauf, Lucas Lombriser and Robert Smith of the University of Zurich and Uros Seljak of the University of California-Berkeley.

The results are important, they said, because they shore up current theories explaining the shape and direction of the universe, including ideas about "dark energy," and dispel some hints from other recent experiments that general relativity may be wrong.

"All of our ideas in astronomy are based on this really enormous extrapolation, so anything we can do to see whether this is right or not on these scales is just enormously important," Gunn said. "It adds another brick to the foundation that underlies what we do."

First published in 1915, Einstein's general theory of relativity remains a pivotal breakthrough in modern physics. It redefined humanity's understanding of the fabric of existence -- gravity, space and time -- and ultimately explained everything from black holes to the Big Bang.

The groundbreaking theory showed that gravity can affect space and time, a key to understanding basic forces of physics and natural phenomena, including the origin of the universe. Shockingly, the flow of time, Einstein said, could be affected by the force of gravity. Clocks located a distance from a large gravitational source will run faster than clocks positioned more closely to that source, Einstein said. For scientists, the theory provides a basis for their understanding of the universe and the foundation for modern research in cosmology.

In recent years, several alternatives to general relativity have been proposed. These modified theories of gravity depart from general relativity on large scales to circumvent the need for dark energy, an elusive force that must exist if the calculations of general relativity balance out. But because these theories were designed to match the predictions of general relativity about the expansion history of the universe, a factor that is central to current cosmological work, it has become crucial to know which theory is correct, or at least represents reality as best as can be approximated.

"We knew we needed to look at the large-scale structure of the universe and the growth of smaller structures composing it over time to find out," Reyes said. The team used data from the Sloan Digital Sky Survey, a long-term, multi-institution telescope project mapping the sky to determine the position and brightness of several hundred million celestial objects.

By calculating the clustering of these galaxies, which stretch nearly one-third of the way to the edge of the universe, and analyzing their velocities and distortion from intervening material, the researchers have shown that Einstein's theory explains the nearby universe better than alternative theories of gravity.

The Princeton scientists studied the effects of gravity on these objects over long periods of time. They observed how this elemental force drives galaxies to clump into larger collections of themselves and how it shapes the expansion of the universe. They also studied the effects of a phenomenon known as "weak" gravitational lensing on galaxies as further evidence.

In weak lensing, matter -- galaxies and groups of galaxies -- that is closer to viewers bends light to change the shape of more distant objects, according to Mandelbaum. The effect is subtle, making viewers feel as if they are looking through a window made of old glass. Studying data collected from telescope surveys of regions showing what the universe looked like 5 billion years ago, the scientists could search for common factors in the distortion of multiple galaxies.

And, because relativity calls for the curvature of space to be equal to the curvature of time, the researchers could calculate whether light was influenced in equal amounts by both, as it should be if general relativity holds true.

"This is the first time this test was carried out at all, so it's a proof of concept," Mandelbaum said. "There are other astronomical surveys planned for the next few years. Now that we know this test works, we will be able to use it with better data that will be available soon to more tightly constrain the theory of gravity."

Astronomers made the discovery a decade ago that the expansion of the universe was speeding up. They attributed this acceleration to dark energy, which they hypothesized pervaded otherwise empty space and exerted a repulsive gravitational force. Dark energy could be a cosmological constant, proposed by Einstein in his theory of general relativity, or it could be a new form of energy whose properties evolve with time.

Firming up the predictive powers of Einstein's theory can help scientists better understand whether current models of the universe make sense, the scientists said.

"Any test we can do in building our confidence in applying these very beautiful theoretical things but which have not been tested on these scales is very important," Gunn said. "It certainly helps when you are trying to do complicated things to understand fundamentals. And this is a very, very, very fundamental thing."

Image 2: Princeton University scientists (from left) Reinabelle Reyes, James Gunn and Rachel Mandelbaum led a team that analyzed more than 70,000 galaxies and demonstrated that the universe -- at least up to a distance of 3.5 billion light years from Earth -- plays by the rules set out by Einstein in his theory of general relativity. Image: Princeton University, Office of Communications, Brian Wilson